Various methods exist for joining two or more metal sheets, panels, or “work pieces” when manufacturing a vehicle or other metal-based products. Typically, such metal work pieces are fused or welded together using a controlled application of intense heat and pressure, with the heat being directed at or along an interface, seam, or joint between adjacent work pieces. While some welding techniques utilize a high-temperature arc in forming the welded joint, other welding techniques apply pressure to the work pieces in order to generate heat using the inherent resistivity or electrical resistance of the metal work pieces. Such a process is referred to generally as “resistance welding”.
Spot welding is one of the more common types of resistance welding techniques, wherein opposing electrodes apply a clamping force on a set of adjacent work pieces while an electrical current is directed through the work pieces. The electrical resistance of the metallic materials forming the work pieces generates intense localized heating, which coupled with the clamping force exerted by the electrodes or welding tips, ultimately melts or fuses the work pieces to form a weld “nugget”. This nugget is positioned within the work pieces themselves, with the external surfaces of a properly formed spot welded joint appearing as a depression or indentation in the surfaces of the work pieces.
Resistance welding techniques such as spot welding can be used in conjunction with a layer of adhesive material which is first applied between the work pieces prior to formation of the joint. The adhesive material provides a portion of the final holding or bond strength. In certain applications, the adhesive material may be cured using any localized heat generated during the resistance welding process. The resultant welded joint is typically stronger than a purely spot-welded joint due to the bonding strength provided by the adhesive material. Moreover, use of the adhesive material can potentially reduce the number of required spot welds for a given application, potentially reducing the overall cost and efficiency of the manufacturing process.
Because of the potential cost and efficiency benefits of weld-bonding relative to traditional resistance welding techniques, modern manufacturing processes are beginning to use weld-bonding more frequently than in the past. However, weld-bonded joints rely heavily on the consistency and integrity of the adhesive material that is used for joining the work pieces. Such adhesive material is commonly epoxy-based, a material which can absorb humidity or moisture from the atmosphere when left exposed in the work environment. When the relative humidity is high, the strength of the joint will usually decline over time. As a result, a weld-bonding process that is employed in a relatively low-humidity environment can produce a welded joint having characteristics that are potentially inconsistent with those of a similar welded joint formed using an otherwise identical weld-bonding process, but formed within a relatively high-humidity environment.